The present invention is directed to an optical grating having a plurality of side-by-side outfeed end faces of a plurality of strip-like optical waveguides for guiding optical waves wherein the optical waves guided in the waveguides can be coupled out of the outfeed end faces and can be overlaid with one another for the formation of a diffraction grating.
A grating of this type is disclosed in an article by M. K. Smit entitled "New Focusing and Dispersive Planar Component Based on an Optical Phased Array", Electron Lett. Mar. 31, 1988, Vol. 24, No. 7, pp. 385-386. In this known grating, thirty-one strip-like optical waveguides are arranged side-by-side. Each of these waveguides is composed of an Al.sub.2 O.sub.3 rib waveguide having a width of 3 .mu.m. The spacing between neighboring rib waveguides is 6 .mu.m. The rib waveguides are conducted along a segment having an aperture angle of 30.degree. on concentric circles having a radius of 910 .mu.m to 1090 .mu.m. Every rib waveguide comprises an infeed end face at one side for infeeding a part of an optical wave to be conducted into the grating and comprises an outfeed end face at the other side for coupling this part out of the waveguide. Since the waveguides are guided on concentric circles along a segment having a defined aperture angle, neighboring waveguides having a difference in their optical length, and the amount of difference is the same for all neighboring waveguides. This difference in the optical lengths of the waveguide causes a mutual phase shift on the part of the optical wave to be coupled into the grating that is carried from the output aperture to the output aperture of the grating. This phase shift is of the same size for all pairs of neighboring waveguides.
The known grating was operated for experimental investigation given a wavelength .lambda. of approximate 0.63 .mu.m for the optical wave to be coupled into the waveguide. According to the published experimental values .DELTA..lambda./.lambda. lies at 0.5%, i.e., .DELTA..lambda. lies at approximately 3.3 nm given a cross talk attenuation of 20 dB and an angle .delta..theta. of approximately 3.degree. between two orders of diffraction that can be read from a photograph so that it cannot be clearly derived whether the recited cross talk attenuation is meant optically or electrically.
Another known optical grating is disclosed in an article by H. Takahashi et al entitled "Arrayed-Waveguide Grating for Wavelength Division Multi/Demultiplexer with Nanometre Resolution" from Electronics Lett., Jan. 18, 1990, Vol. 26, No. 2, pp. 87-88. In this grating, the plurality of strip-like optical waveguides are arranged side-by-side and are composed of composite monomode optical waveguides that are fashioned in a base layer of glass and are manufactured with a traditional photolithic technique. These waveguides have different lengths and bent sections having the same radius. Also in this known grating, a waveguide comprises an infeed end face at one side for the infeed of an optical wave to be coupled into the grating and comprises an outfeed end face at the other side for coupling this part out of the waveguide. The difference between the optical lengths of two neighboring waveguides is of the same size for all neighboring waveguides so that the parts of the optical wave coupled into the grating that are carried in the waveguides given this known grating also have a mutual phase shift from output aperture to output aperture that is the same size for all pairs of neighboring waveguides.
In an exemplary embodiment of this known grating, one hundred and fifty-five waveguides, each having a cross sectional dimension of 1.2 .mu.m.times.1.5 .mu.m are integrated on a substrate. The spacing between respective two neighboring waveguides amounts to 8 .mu.m and the radius of every bent section is selected at 1 mm. Neighboring waveguides differ by 17.54 .mu.m in length. A wavelength resolution of less than 1 nm can be obtained with this exemplary embodiment. The optical wave to be coupled into the grating is coupled into the one hundred and fifty-five waveguides via a cylindrical lens. Given a wavelength of 1.3 .mu.m, the wavelength resolution was capable of being experimentally set at 0.63 nm.